On 2/12/23 23:01, Markus Schönhaber wrote:
02.12.23, 11:33 +0100, fedora@xxxxxxxxxxxxxx:
For the last few weeks I am trying to pinpoint the source of a network
problem.
The short story: using the standard mtu of 1500 caused sending to stall
(and fail).
This is seen when uploading a file (e.g. ftp) or when sending email.
With an mtu of 1456 everything works.
Everything worked unto 02:30 on 7/Nov. At that point some uploads
started hanging.
I have cron scripts that upload small and larger files. The small ones
get though, the larger ones hang.
There is enough space on the target.
Assuming that the problem's not on your side of the network, the most
likely cause I can think of, is that now somewhere along the path ICMP
packets signalling the need for fragmentation get dropped, and therefore
the sender doesn't learn that it should send smaller packets.
You might want to double check that your network doesn't behave that way.
I will double check, though the network worked well until then, so I cannot see
how my side is the cause. The system was rebooted since and the problem persisted.
The network is simple: my server connects to a modem (wireless internet
over 4G).
BTW, is there a more suitable list (or forum) for discussing this issue?
I want to confirm where the problem lies. I did not change a thing on my
server.
I suspect that the modem updated its fw overnight and the new fw has an
issue.
However the ISP says no one else is complaining.
I did not find a way to downgrade the fw or install the old one.
I was given a new modem (with the old fw) but once connected to the 'net
the fw was updated, something I cannot control.
If the update of the modem's FW happened around the same time the
problem arose, that would point to the new FW as being the culprit
(maybe by dropping those "evil" ICMP packets as I mused above).
And as long as you're reasonably sure that the problem isn't caused by
some change *you* made, and you can't control the modem's behavior, you
could simply accept your fate, lower the interface MTU and move on.
I did not do any change to the firewall, which is probably the only place
I could break things (like dropping ICMP).
The modem has a feature "IPv4 SPI Firewall" and turning it off did not help here.
Anyway, if icmp if dropped then I expect simple pings will not get though either, right?
BTW: if your modem does PPPoE (as is common on DSL lines) it might not
be needed to lower the MTU below 1492 since the PPP header occupies only
8 bytes of the ethernet frame's payload.
No, I actually tried and any higher than 1456 and it fails.
This is a wireless (4G) internet modem.
Below is an example of the issue.
$ sudo ifconfig eth1 mtu 1500 # this is the default, which
now fails
$ ifconfig eth1
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 192.168.2.7 netmask 255.255.255.0 broadcast 192.168.2.255
$ tracepath -4 -b -l 1500 -m 5 1.1.1.1
1: e5.eyal.emu.id.au (192.168.2.5) 1.109ms #
this is the modem
2: no reply
3: no reply
4: no reply
5: no reply
Too many hops: pmtu 1500
Resume: pmtu 1500
## Does the above show that the server (.7) can talk to the modem (.5)
but the modem
## cannot talk to the external net?
## Can it be an issue with the next hop (the 4G service)?
$ tracepath -4 -b -l 1456 -m 5 1.1.1.1
1: e5.eyal.emu.id.au (192.168.2.5) 0.760ms
2: 10.155.144.3 (10.155.144.3) 74.809ms
3: 10.246.20.29 (10.246.20.29) 39.707ms
4: 101.115.95.250 (101.115.95.250) 38.656ms
5: 203-220-16-149.tpgi.com.au (203.220.16.149) 35.127ms
Too many hops: pmtu 1456
Resume: pmtu 1456
$ sudo ifconfig eth1 mtu 1456 # with this everything works well
$ ifconfig eth1
eth1: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1456
inet 192.168.2.7 netmask 255.255.255.0 broadcast 192.168.2.255
$ tracepath -4 -b -l 1500 -m 5 1.1.1.1
1?: [LOCALHOST] pmtu 1456
1: e5.eyal.emu.id.au (192.168.2.5) 0.797ms
1: e5.eyal.emu.id.au (192.168.2.5) 0.614ms
2: 10.155.144.2 (10.155.144.2) 37.951ms
3: 10.246.20.29 (10.246.20.29) 36.881ms
4: 101.115.95.249 (101.115.95.249) 35.653ms
5: 203-220-16-145.tpgi.com.au (203.220.16.145) 36.673ms
Too many hops: pmtu 1456
Resume: pmtu 1456
$ tracepath -4 -b -l 1456 -m 5 1.1.1.1
1: e5.eyal.emu.id.au (192.168.2.5) 0.560ms
2: 10.155.144.2 (10.155.144.2) 33.008ms
3: 10.246.20.29 (10.246.20.29) 36.226ms
4: 101.115.95.250 (101.115.95.250) 36.278ms
5: 203-220-16-149.tpgi.com.au (203.220.16.149) 36.156ms
Too many hops: pmtu 1456
Resume: pmtu 1456
--
Eyal at Home (fedora@xxxxxxxxxxxxxx)
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